I was having the same problem with Opera 49.0.2725.47 (I was sure my details - username and password - were correct) and I couldn't find a fix for it. However I 've just installed Opera Developer (Opera Beta 50.0.2762.18) and I can see all the passwords without a problem using the email and password I use to log in Windows 10.
Well... The problem is not fixed, but it's a temporary solution.